Rick Denney wrote:
> Ed writes...
>
>> You need to measure this for forward/reverse resistance, not continuity.
>> This diode acts as a fuse to protect the metering circuit. With no drive
>> applied, you should see no grid current.
>
> Bingo! D16 was a dead short. I replaced it, and the amp now seems to
> work fine.
Good deal.....Good job...keep in mind this also is in the B- return line
and it needs to "float" above ground to complete the cicuit.
>> Another thing that bothers me is the ALC circuit. again an old and
>> proven design. However it picks off some of the RF output, rectifies
>> it and sends it back to the input circuit. If it were me I would
>> take this out of the RF path, especially since TT does not provide
>> ALC.
>
> If I end up with a Centaur, this will be my second operating position
> when I install a new antenna later this year, and that radio (a
> Kenwood) has ALC.
Still not needed. ALC was used years ago when most transmitters were
capable of running in excess of 100 watts.
>> Keep in mind that TT stopped producing the Centaur, not because it
>> didn't sell, but they could not find acceptable tubes.
>
> What is meant by "acceptable"? Does this mean that the Centaur has
> different requirements than the Ameritron in terms of tubes? I figured
> I could just order the Chinese-made Taylor Select tubes from RF Parts,
> and not drive them as hard.
No the requirements are the same. The Chinese tubes are too un-reliable
and finding 2 with the same operating characteristics are next to
impossible to find. The ideal situation would be 3 matched tubes. If it
were me, I would look for NOS Jan tubes. And forget the 572's, there is
no advantage other than a minimal increase in output.
>> I'm not going to comment on the parasitic discussion other than to say
>> that most catastrophic events in an amp are caused by component failure
>> and/or operator error.
>
> When tuning on 10 meters, I overdrove the grid and plate currents. I
> was looking at the wrong meter (the wattmeter) and overdrove the amp
> during tuning. Also, for a while now I've been tuning the amp using
> the Tune control, and switching the amp with the Relay out from the
> Omni V. This combination apparently hot-switches the amp. I suspect it
> was the overdriving that caused the problem. I wonder if the old
> suppressor design's inductor wasn't bypassing all that excessive 28
> MHz RF around the resistor, causing it to overheat and blow. For all I
> know it may have already been blown, waiting for me to use it on 10
> (which I had not done before).
>
> Anything I should look at carefully before declaring victory?
>
>> **** KEEP IN MIND HV CAN AND WILL KILL YOU **** BE CAREFUL ****
>
> Oh, yes.
>
> Thanks for your excellent help.
>
> Rick, KR9D
I would recommend tuning the amp with some type of pulser arrangement.
The best method is to run your keyer up to 40+ wpm and tune the amp in
the CW position. Or you could build/buy a pulser unit for SSB. Do not
tune the amp at full out from the rig, 50-60 watts drive is plenty on 811s'.
There are several circuits around that will prevent the hot switching. I
think Harbach sold one at one time as well as Rick Measures, but I'm not
real sure on that. Also check the TT archieves as I know this has been
discussed before.
Last piece of advice before I go eat breakfast. If you plan on repairing
your amps, get an ARRL or Bill Orr handbook and study up on the theory
of how an amp works. Take a look at some of the circuits and what they
do and how they operate. A dead amp with no visible signs of damage is a
difficult animal to fix.
Glad I was able to help....I enjoy giving back to Ham Radio for what it
has given me for the last 30 years........73
Ed W3NR
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